Once you add the Flat File Listener the configuration tool will looks like below:

After that select the log file name to log the events and the Formatter as Text Formatter.

Upon making the above settings make sure to add the above listener to the All Events settings as below :

Upon following the above steps write the following code in C# and test the solution, you will find the entries written upon in the Event Log and the Log file which you did specified in the above step :